Amplifier



N. E. LINDENBLAD 2,131,566

AMPLIFIER Filed Feb. l, 1935 3 Sheets-Sheet l Sept. 27, =19.38.

Sept. 27,- 1938. N. E. LINISENBLAD AMPLIFIER Filed Feb. .1, 1955.

FREQUENCY P/WH HIGH FREQUENCY PA TH IIIl PATH Ffa-@amer l PA TH INVENTOR. NILS E. LINDENBLAD 7 ATTORNEY.

Sept. 27, 1938. N. E. LINDENBLAD lAMPLIFIER 3 Sheets-Sheet 3 Filed Feb. 1,- 1935 Qm w, W w M,

Patented Sept. 27, 1938 PATENT OFFICE AMPLIFIER Nils E. Lindenblad, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application February 1, 1935, Serial No. 4,474

9 Claims.

This invention relates to amplifiers of the thermionic type and is in particular concerned with an amplifier of flat characteristics over an extremely wi-de frequency range such as for example 20 to 1,000,000 cycles. The amplifier of the present invention makes use of thermionic tubes now in commercial use, although certain characteristics of said tubes, such as limited electron emission and large inter-electrode capacity,

i0 work against the operation of said tubes effectively in known circuits over a frequency band of less width than the frequency band involved in this invention.v By my novel circuit, a frequency band of extreme width is successfully amplified,

and all of the frequencies in said band are amplified substantially the same amount.

In working on this problem, it at once became clear that transformer coupling between the stages must be excluded at least for the time being, because of the limited frequency range which can be passed by transformer coupling means. I next investigated the possibilities of capacity-resistance coupling. Moreover, it was found desirable to use such coupling in conjunction with push-pull type of amplifier circuits due to their symmetry and the ease with which neutralizing may be accomplished in such circuits. Having deci-ded on these points, the task left was to determine such coupling constants which would result in a flat amplifying characteristic between 20 to 1,000,000 cycles. It became apparent that a condenser with a high `coupling capacity and having the necessary insulation to separate the plate voltage in one stage from the grid voltage in the next stage is of necessity a condenser of appreciable physical dimensions. A condenser of such dimensions has an appreciable stray capacity,-while this stray capacity has only a small shunting effect at 20 cycles, its shunting effect at 1 megacycle has a paralyzing iniiuence on the efforts to obtain the desired fiat characteristic. On the other hand, if a condenser of smaller physical dimensions be used, it cannot be given enough capacity to carry through the lower frequencies. It `also became clear that a compromise could hardly be obtained and that instead of a compromise, some kind of combination of the two condensers had to be worked out in order to obtain the desired result. The

solution that was finally arrived at was to combine one small and one large capacity as components in a network or impedance trap of such a characteristic that practically only one of the two capacities are in function at each extreme end of the frequency band. Both capacities are to Avaried extents operative at frequenciesv intermediate the extreme ranges. n

As indicated above, the amplifier of the present invention is, for reasons also pointed out above, of the push-pull type. The input energy l5 supplied to the amplifier may be supplie-d from a push-pull source and may be fed in the usual manner to the control grids'of the first stage. y In many cases, it is necessary to connect the amplifier of the present invention to a single- 10 ended stage. Several novel means for adapting thepush-pull wide frequency band amplifier of the present invention to connection with a singleended source have been disclosed in this application. v

' The nature of my invention and the manner in which the same is carried out will be understood from the following detailed descriptionthereof and therefor when read in connection with the drawings throughout in which like characters 20 indicate like parts insofar as possible and i which:

' Fig. 1 illustrates a thermionic amplifier of the push-pull type comprising several stages interconnected by the novel network of the present 25 invention. In Fig. 1, the input is assumed to be taken from a push-pull source, while the output circuit ofthe final stage is arranged to feed either a push-pull utilization circuit or a vsingleended circuit. 30

Figs-2 andv 3- illustrate amplifiers of the nature illustrated in Fig. 1. In Figs. 2 and 3, however, a'novel means is included for feeding the push-A pull amplifier from the output of a single tube or single-ended stage. 35

Fig. 4 is a curve illustrating the characteristics of the amplifier of the present invention.

In Fig 1, I have shown a cascaded-ampliiie-r of the push-pull type, comprising pairs of electron discharge tubes 2, 4, 2', 4 etc., connected in 40 cascade by way of resistancefand capacity networks, as shown. The input of the first `stage l may beconnected to a full wave source. The anode-to-control grid capacity of each stage is as shown neutralized by a capacity NC. The 45 output of thepush-pull amplifier may be connected to a push-pull utilization circuit by means of leads represented by the solid lines or to a ysingle-ended input by means of one of the said vleads and a lead represented by the dottedline. 50

As willibe seen by an inspection of Fig. l, the small capacities C1 are directly connected between the anodes ofthe tubes 2, 4 and the kcontrol grids of the tubes 4', 2, respectively, whereas the large capacities C2 areas shown connected in55 parallel to the capacity C1 by way of link resistors R1. The succeeding stages are as shown connected in a similar manner.

During operation at the high frequency end of the band the small capacities C1 have a low reactance and therefore the coupling current prefers passage through these capacities rather than going through capacitors C2 by way of link resistors R1. This is, however, not a point of importance. The important fact is that at this high frequency, the stray capacity of the large condenser C2 is in series with the resistors R1, and the eiect of this stray capacity on the swing of the grids is reduced. At the very lowest frequencies, the reactance of condensers C1 is very high and the coupling current is forced through condensers C2 by way of the resistors R1. This might seem inecient to force the coupling current through a resistance, but it must be remembered that the tube reactance being extremely high at these frequencies results in a very high grid impedance, since the grids are biased to carry no electron current. The fact that the load from the grids is negligible at the low frequencies therefore makes the fact insignificant that they are fed through resistors R1. ments of the characteristics of this circuit, it was found that it was not necessary to take too great precautions in keeping the stray capacities of condensers C2 down by providing very spacious shields S around the said capacities. The spacing of the shields was made smaller and this is desirable as it results in smaller dimensions of the amplifier as a whole. The stray capacities of these condensers not only can be made larger, but it was found advisable and necessary to d so. To increase the stray capacities of the condensers C2, I provide as shown, an additional condenser C3 which is preferably variable. This condenser, which is small in capacity relative to condenser C2, may be connected on either side of C2 between C2 and ground. In this manner, a iine regulation of the stray capacity may be accomplished. Another reason for the'need of the variable capacity C3 is that at the intermediate frequencies of about 100,000 cycles, the amplifier without the use of said capacities gives somewhat higher amplification due to an optimum impedance condition of my simple network without a correct stray capacity of condenser Cz. With this condition corrected by means of adjusting the stray capacity, the variation in amplication throughout the Whole band between 20 cycles and 1 megacycle was at one time brought below two decibels (125:1).

In operation, it is desirable to have a well regulated or steady direct current source B supplying plate current to this type of amplier. Preferably, however, a separate voltage regulator tube RT and the resistor system RS may be connected as shown with the source B. A similar arrangement has been described in detail in my United States application #603,310 filed April 5, 1932 now Patent #2,052,576` dated September l, 1936. The source B may be a direct current machine or may represent the output oi a-'rectifier connected with a source of alternating current. Grid biasing potentials for the tubes 2', 4, and 2", 4" may be supplied from sources connected as shown with the control grids of these tubes. Grid biasing potentials for tubes 2 and 4 may be supplied by the source connected with the cathodes of 2 and 4 by way of a lead (not shown) connected to a point on the input circuit.

In practice, it is desirable that terminal cir- By actual measure- .with the plate potential source B by way of a recuits be arranged so that this amplifier, which as shown in Fig. 1 is assumed to be supplied from a push-pull stage, may be connected also to single ended circuits in such a way that the push-pull stages receive balanced input. An arrangement 5 for accomplishing this has been shown in Fig. 2. Assume that the signal is to be supplied from the output of a single ended stage between the control grids of on-e of the tubes 2, 4 and ground as shown by way of the leads marked Input; the l0 signal on the grid of tube 2 modulates or controls the voltage on the plate of tube 2 and this control or variation is opposite in phase to the controlling potentials applied t0 the grid. This voltage variation or modulation appears across the resistor X connected as shown by way of source Y between the plate of tube 2 and ground. Voltage variations ror modulations are supplied from the resistance or potentiometer X by way of a movable point and a lead to the control grid of tube 4. By moving the point on the potentiometer X to the proper position, voltages of an amplitude equal to the amplitude of the voltages on the grid of tube 2 and of a phase opposite in phase to the voltages on the grid of tube 2 may be applied to 25 the grid of tube 4 so that we have a true pushpull input. The source Y provides a counter electromotive force to insure correct direct current voltage on the grid of tube 4. Condensers proportional, in reactance, to the resistance on each side of the tapping point may be connected with the resistance X as shown. These condensers, if used must be very small. An additional circuit containing a resistor X may be added to the plate of tube 4 to complete the balance.-

Another arrangement for adapting a single ended input to my push-pull amplier has been shown in Fig. 3. In Fig. 3, a converter tube CT may have its control grid and cathode connected as shown to any source of alternating current po- Vtential to be ampliiied. A capacity may be included in this connection. The input line and the control grid of tubeY CT yare also connected as ,shown by way of a coupling and direct current blocking condenser C5 to the control grid of one of the tubes in the rst push-pull stage, as for example tube 2. The grid of the other tube, as for example tube 4, in the iirst push-pull stage -isvsupplied by potentials from a potentiometer resistance X connected in series between the plate and the anode source B. The variations of the voltage across this potentiometer will then be of opposite phase with respect to the voltage variations applied to the grid of this same converter tube CT. By adjusting the tap connected between the control grid of tube 4 and potentiometer resistance X by way of coupling and direct current blocking condenser C4, a voltage for the grid of tube 4 may be obtained which is equal to the volt-age applied to the grid of tube 2 and of a proper phase relative to said voltage to insure balanced push-pull input for the push-pull stage 2, 4. The point at which the condenser C4 is connected to X depends upon the amplication ofthe tube CT. 65 Biasing potentials to the control -grids of tubes CT, v4, and 2 respectively from the source BS -are supplied through the resistances R3, R4, and

R5. The regulator tube RT may be connected .'70 sistance system RS, as shown.

The push-'pull amplifier of Figs. 2 and 3 and the networks interconnecting the stages thereof are as shown in Fig. 1.

The characteristics of a push-pull amplifier the ..75

cascaded stages of which are interconected by the network of my invention have been illustrated by the curves in Fig. 4, wherein frequency iS plotted as abscissae against Voltage amplification as ordinates. As can be seen, the amplifier has a iiat characteristic over an extremely wide frequency range.

I claim:

l. In a circuit for transferring potentials the periodicity of which may vary over a wide range, a conductor which may be energized by said potentials, a coupling condenser in series with said conductor, said condenser being of low impedance to potentials the frequency of which fall at the upper end of said range, a second coupling condenser being of low impedance topotentials at the lower end of said range, and an impedance connecting said second coupling condenser in` parallel with said rst coupling condenser whereby potentials over said entire range are affected as to amplitude in substantially like manner by said couplings and appear at the output end of said conductor.

2. In a balanced full wave or push-pull circuit for transferring potentials the periodicity of which may vary over a wide range, a pair of con'- ductors each having one terminal which may be energized by said full wave potentials, a coupling condenser in series with each of said conductors, said coupling condensers being of low impedance to potentials the frequency of which fall at the upper end of said range, a second coupling condenser of low impedance to potentials at the lower end of said range for each of said conductors, and resistive means connecting one of said second named condensers in parallel with each of said rst named coupling condensers, whereby potentials over said entire range are affected as to amplitude in substantially like manner while being passed by said coupling condensers and appear at the output ends of said conductors.

3. In a circuit for transferring potentials the periodicity of which may vary over a wide range from the anode of one tube to the grid of a second tube, a. coupling condenser having one terminal coupled to said anode and the other terminal connected to said grid, said coupling condenser being of low impedance to potentials the frequency of which fall at the upper end of said range, a second coupling condenser of low impedance to potentials at the lower end of said range, and impedance means connecting said second coupling condenser in parallel with said rst coupling condenser whereby potentials over said entire range are transferred in like manner between said anode and grid.

4. In a full wave circuit for transferring potentials the periodicity of which may vary over a wide range from the anodes of a pair of electron discharge tubes to the control grids of a second pair of electron discharge tubes, a coupling condenser connected between the anode of each of said tubes of said first pair and the control grid of aV tube of said second pair, said coupling condensers being of low impedance to potentials the frequency of which fall at the up-V per end of said range, and an additional coupling condenser/of loW impedance to potentials at the lower end of said range coupled by resistive means in parallel with each of said rst named coupling condensers, whereby potentials over said entire range are transferred in a linear manner from the anodes of said iirst named pair of tubes to the control grids of said second named pair of tubes. 5. In a signalling system, pairs of electron discharge tubes each having input and output electrodes, including an. electron emission element, connected in push-pull relation by means of neutralized circuits, means connected with the input electrodes of one pair of said tubes for impressing thereon varying potentials of a wide frequency range to be relayed, a pair of condensers of substantially like value coupling the anode of each of the tubes of said oneI pair of tubes to the input electrode of a. different one lof the tubes in another of saidpairs of tubes, said condensers being of low impedance to potentials, the frequencies of which fall at the upper end of said range, and a condenser of diiferentcvalue connected by an impedance in parallel with each of said aforesaid condensers, said condensers of different value being of low impedance to potentials the frequency of which falls at lthe lower end of said range, said last named condensers of differ-A ent value also being of substantially like Value.

6. A signalling system as recited in claim 5 in Y which said impedances are resistive in character .and of substantially linear impedance to all of the potentials of said wide frequency range.

7. A system as recited in claim 5 wherein a reactance which is Variable connects one terminal of at least one of said last named condensers to the emission elements of said last named pair Off tubes.

8. A system as recited in claim 1 wherein a Variable condenser is connected in parallel with said second condenser.

9. A system as'recited in claim 3 in which a variable condenser is connected between one terminal of said second' named condenser and the cathode of said one of said tubes.

NlLS E. LINDEN'BLAD. 

